Rotary print drum having an intermittent drive

ABSTRACT

A rotary print drum has a series of circumferentially spacedapart rows of type on the periphery thereof and a drive system effects intermittent rotation of the print drum to successively position the rows of type in position to effect a printing operation. The drive system comprises a ratchet wheel connected to the print drum, a pawl, and an electromagnet for reciprocating the pawl to effect intermittent rotation of the print drum. Detecting means detects when the print drum is in a prescribed angular position and provides a corresponding output signal. A control circuit applies successive pulse signals to the electromagnet to reciprocate the pawl and responds to the output signal from the detecting means to terminate the application of pulse signals to the electromagnet only after the print drum has been angularly driven through a predetermined number of angular increments.

United States Patent Chida et al.

- Jan. 21,1975

[ ROTARY PRINT DRUM HAVING AN INTERMITTENT DRIVE 3,663,877 5/1972 Clark 318/254 Primary Examiner-G. R. Simmons I [75] Inventors f gi gg g i z fii gg Attorney, Agent, or Firm Robert E. Burns;

Emmanuel .1. Lobato; Bruce L. Adams [73] Assignee: Kabushiki Kaisha Seikosha, Tokyo,

Japan [57] ABSTRACT Filedi g- 1972 A rotary print drum has a series of circumfercntially spaced-apart rows of type on the periphery thereof [21] Appl 278470 and a drive system effects intermittent rotation of the print drum to successively position the rows of type in [52] US C 101/9312, 8/6 5 position to effect a printing operation. The drive sys- [51] Int. Cl B4lj 23/34 tem comprises a ratchet wheel connected to the print [5 F d Of Search 0/ 0, 37; 318/138, 254, drum, a pawl, and an electromagnet for reciprocating 318/4 ,685; 101/93, 93 C; 94/527 the pawl to effect intermittent rotation of the print drum. Detecting means detects when the print drum is [56] References Cited in a prescribed angular position and provides a corre- UNITED STATES PATENTS sponding output signal. A control circuit applies sue- 3 010055 11/1961 Nicolaus 310/23 X cessive pulse signals to the electromagnet to recipro- 3:332:068 7/1967 McLaughlin 5615. It: 507/93 0 Cate the pawl and responds to the P Signal from 3,359,474 12/1967 Welch et a1 318/254 x the detecting means to terminate the application of 3,374,410 3/1968 Cronquist et a1... 318/254 pulse signals to the electromagnet only after the print 3,423,658 1/1969 Barrus 318/696 drum has been angularly driven through a predeter 3,500,103 3/1970 Swain et a1. 318/254 mined number of angular increments, 3,523,230 8/1970 York i 318/138 3,586,953 6 1971 Markkanen 318/685 7 Clalms, 2 Drawmg Flgures 22 64 ZF MFA/1A5 /5 PULSE Z5 1 5/6 61: [[[CTRO- l 1 man/U l 1 24 {26 1 1 PR/A T 6 OMMAA/D T 1 UR l I 5/6/1041 Rd ROTARY PRINT DRUM HAVING AN INTERMITTENT DRIVE The present invention relates to a printing mechanism and more particularly to a printing mechanism having a rotary print drum and means for rotationally driving the drum in an intermittent manner.

In conventional printing mechanisms, a continuously operating motor is employed as the power source for rotationally driving a rotary print drum. The print drum contains a plurality of rows of print type angularly spaced around the periphery thereof and a row of print hammers is positioned along the print drum and are selectively actuated to strike desired ones of the rows of type to effect a printing operation. A paper strip is advanced between the print drum and the print hammers and the motor rotates the print drum either continuously or intermittently to successively position the various rows of type in aligned opposition to the print hammers.

The disadvantage of the prior art printing mechanisms is that the motor occupies considerable space within the printing mechanism and thus it is not possible to construct the printing mechanism as a miniature small-sized unit. The motor is an expensive component and consequently the price of the printing mechanism must reflect the cost of the motor.

In those printing mechanisms wherein the print drum is continuously rotated, an angle detecting device is necessary in order to detect the angular position of the print drum to effect actuation of the print hammers in synchronization with the rotation of the print drum. The angle detecting devices heretofore employed are complex structures requiring precise manufacturing accuracy, frequent matainence and adjustment, and therefore require much labor and expense.

In the printing mechanisms wherein the print drum is intermittently rotated, some type of mechanical lostmotion device must be coupled to the motor in order to convert the continuous rotation of the motor output shaft into an intermittent rotation in order to intermittently drive the print drum. Thus 21 Geneva mechanism or other similarly functioning mechanism must be included within the printing mechanism and such renders the mechanism more complex and expensive. Moreover, some device must be provided for coordinating the intermittent motion of the print drum with the actuation of the print hammers.

It is therefore a primary object of the present invention to provide a printing mechanism which is small in size, inexpensive to manufacture, reliable in operation, and has a high printing accuracy for use in a desk-type electronic calculator, a measuring instrument or a terminal device of a communication machine.

It is another object of the present invention to provide a printing mechanism having an electric circuit for controlling the printing operation and for preventing a misprinting operation.

It is a still further object of the present invention to provide a printing mechanism utilizing an electromagnetically driven ratchet wheel as the power source instead of a motor therefore simplifying the structural composition of the mechanism.

The above and other objects of the present invention are carried out by a printing mechanism comprising a rotary print drum having on the periphery thereof a series of angularly spaced-apart rows of print type, a

ratchet wheel connected to the print drum, a pawl in engagement with the ratchet wheel, electromagnetic driving means for reciprocally driving the pawl to effect intermittent rotational movement of the ratchet wheel, detecting means for detecting a predetermined angular position of the print drum and providing a corresponding output signal, and a control circuit for controlling the energization of the electromagnetic drive means and responsive to the output signal to terminate the driving of the print drum. The control circuit includes a counter actuated by the output signal to count incremental angular movements of the print drum and the counter delivers a reset signal to a gate circuit to effect termination of the driving of the print drum when the same has angularly displaced through a predetermined angle.

Having in mind the above and other objects that will be evident from an understanding of the disclosure, the present invention comprises the combinations and arrangements of parts illustrated in the presently preferred embodiment of the invention which is hereinafter set forth in sufficient detail to enable those persons skilled in the art to clearly understand the function operation, construction, and advantages of it when read in conjunction with the accompanying drawings wherein like reference characters denote like parts in the various views and wherein:

FIG. 1 is an elevational perspective view of the mechanical components of the printing mechanism according to the present invention, and

FIG. 2 is a block diagram ofa control circuit for controlling the actuation of the printing mechanism shown in FIG. 1.

One embodiment of a printing mechanism constructed in accordance with the principles of the invention is shown in FIG. 1 and comprises a print drum I mounted on a rotary shaft 2. The periphery of the print drum 1 is divided into n number of equal circumferentially spaced-apart segments and a set of print type extends longitudinally along the length of the drum in each segment. In the present embodiment the n number of sets of print type comprise 16 rows of print type and thus the print drum is provided with 16 angularly spaced-apart rows of type and may effect the printing of 16 separate and distinct bits information depending upon the angular position of the print drum.

The printing mechanism also includes a row of print hammers (not shown in the drawings) arranged along the print drum in parallel therewith and individually mounted for pivotal movement towards and away from the print drum 1. Each print hammer has a hammer head for striking individual ones of the types disposed along any one of the rows of type on the print drum 1. The print drum is intermittently rotated by driving means which will be described hereinafter to intermit tently position successive rows of type in opposition of the row of print hammers whereby the print hammers may be individually actuated to strike the desired row of type.

A paper strip is intermittently advanced between the print hammers and the print drum and the information contained on the desired row of type is printed on the paper strip by the coaction of the hammer heads striking the row of type with the paper 'strip contained therebetween. It is understood that the paper strip is advanced one increment per each revolution of the print drum 1 and thus any of the 16 rows of type or selected portions thereof may be printed on each successive line of print of the paper strip. The mechanisms for advancing the paper strip and actuating the print hammers are known in the art and do not constitute part of the present invention and therefore will not be further described. 1

The driving means for intermittently driving the print drum in angular increments comprises a ratchet wheel 3 connected to one end of the shaft 2 and having around the periphery thereof 16 similarly dimensioned ratchet teeth. A driving lever 4 is mounted for pivotal movement about a pin 5 affixed to a frame portion of the printing mechanism. A pawl 6 is pivotally mounted on the distal end of the driving lever 4 by means of a pin 7. The pawl 6 is biased into constant contact with one of the ratchet teeth by a biasing spring 8 and another biasing spring 9 continuously urges the driving lever 4 to an initial limit position wherein the tip of the pawl 6 rests upona flank portion of one of the ratchet teeth as shown in FIG. 1.

A stop member 10 defines the initial limit position of the driving lever 4 wherein the driving lever abuts against the stop member 10 and is maintained thereagainst by the biasing spring 9. Another stop member 11 defines the terminal limit position of the driving lever 4 wherein the stop member 11 abuts against a surface portion of the pawl 6 to limit the extent of clockwise movement of the driving lever 4 effected by the actuating means described below. Thus it may be seen that the drive lever 4 is mounted for pivotal movement between two limit positions defined by the stop members l0 and 11.

Actuating means 12, 13 effects reciprocal pivotal movement of the drive lever 4 between the two limit positions. The actuating means comprises an armature l2 affixed to the drive lever 4 at a location adjacent the pin 5 and an electromagnet 13 is affixed to a frame portion of the printing mechanism in opposed relationship from the armature 12. When the electromagnet I3 is energized, a magnetic field is created therearound which coacts with the armature 12 to attract the armature 12 towards the electromagnet thereby pivoting the drive lever 4 in a clockwise direction. The gap spacing between the armature l2 and the electromagnet 13 is suitably chosen in relation to the pitch of each two adjoining ratchet teeth such that movement of the drive lever from the initial limit position shown in FIG. 1 to the terminal limit position wherein the pawl 6 abuts against the stop member 11 causes the pawl 6 to angularly advance the ratchet wheel 3 a distance equal to one ratchet tooth thereby advancing the print drum one/sixteenth of a revolution. When the electromagnet 13 is deenergized, the magnetic field collapses and the force of attraction between the armature l2 and the electromagnet terminates afterwhich the biasing spring 9 urges the drive lever 4 in a counterclockwise direction back to the initial limit position.

Detecting means A is provided for detecting a predetermined degree of angular displacement of the print drum 1 and providing and output signal indicating that the print drum has angularly advanced the predetermined distance. The detecting means comprises a cam 14 affixed to the shaft 2 for movement therewith and the cam 14 has a camming projection 15. The camming projection 15 subtends a central angle corresponding to two adjacent ratchet teeth. A detecting lever is pivotally mounted on a pin 17 for pivotal rocking movement about the pin 17. On one end of the detecting lever 16 is provided a cam follower 18 and a biasing spring 19 is connected to the other actuating-end ofthe detecting lever 16 and urges the detecting lever in a counterclockwise direction to continuously urge the cam follower 18 into engagement with the cam 14.

A switch 20 is mounted beneath the actuating-end of the detecting member 16 and the switch has a movable contact pin engageable with that end of the detecting lever 16 depending upon the position of the lever. In the embodiment shown, the switch 20 is turned ON and provides an output signal when the movable contact pin is in its fully extended position (which occurs when the detecting lever is pivoted in a counterclockwise direction and assumes the position shown in FIG. 1 by phantom lines) and is turned OFF providing no output signal when the movable contact pin is depressed (which occurs when the detecting lever 16 is in the position shown by solid lines in FIG. 1).

In the embodiment disclosed, the camming projection 15 coacts with the cam follower 18 to pivot the detecting lever 16 in the clockwise direction to turn the switch 20 OFF during two/sixteenths of one rotational cycle. Thus the detecting means provides no output signal whenever the print drum I is angularly displaced to the position shown in FIG. 1 and this condition is main tained for two successive stepwise movements of the print drum 1 due to the fact that the camming projec tion 18 subtends a central angle equal to two ratchet teeth. During the third stepwise movement of the print drum 1, the cam follower l8 rides off the camming projection 15 whereupon the biasing spring 19 pivots the detecting lever 16 in a counterclockwise direction to the phantom line position. The detecting lever 16 is thus moved out of engagement with the movable contact pin of the switch 20 thereby turning the switch ON.

It is understood that the switch 20 may be replaced by a conventional microswitch, reed switch or the like without departing from the present invention.

FIG. 2 disclosed a block diagram of one embodiment of the circuitry used in conjunction with the printing mechanism disclosed in FIG. I. First the circuitry for applying actuating pulse signals to the electromagnet 13 will be described and then the circuitry for terminaling the application of actuating pulse signals to the electromagnet will be described.

A pulse signal generator 22 delivers an output pulse signal composed of a succession of pulses having a certain pulse repetition rate. The pulses are fed into a gate means B and the gate means selectively gates the pulses to the electromagnet 13 to intermittently energize the electromagnet. The gate means B comprises an AND gate 23 having two inputs, one of which receives the output pulse signal from the pulse signal 22 and the other of which receives a gate signal from a flip-flop 25.

The flip-flop 25 responds to a print command signal 24 and accordingly delivers a gate signal to the AND gate 23 to open this gate and allow the output pulse signal from the generator 22 to be applied to the electromagnet 13. Thus in the absence of a print command signal, the pulse signal from the pulse signal generator 22 is blocked by the gate 23 and the electromagnet 13 remains deenergized. The print command signal 24 is delivered from a control device not shown in the drawings such as a control device in an electronic ealculator.

The circuitry for terminating the application of pulses to the electromagnet 13 comprises a counter 26 connected to the output terminal of the AND gate 23 for counting the number of pulses delivered from the AND gate when the counter is turned ON. An output terminal of the counter 26 is connected to the reset terminal of the flip-flop 25 and the counter delivers an output reset signal to the flip-flop 25 to reset same in response to counting a predetermined number of output pulses from the AND gate 23. The counter 26 is turned ON and commences counting the output pulses by the output signal from the switch 20. Thus during the time that the switch 20 is turned OFF, the counter 26 does not count any pulses from the AND gate 23 as soon as the switch 20 is turned ON, the counter counts a predetermined number of pulses and then delivers a reset signal to the flip-flop 25 to reset same.

Each pulse from the pulse signal generator 22 which passes through the AND gate 23 is sufficient to momentarily energize the electromagnet l3 and accordingly actuate the drive lever 4 to angularly drive the print drum one/sixteenth of a revolution. Thus, 16 successive output pulses are necessary to effect one complete revolution of the print drum.

The operation of the printing mechanism will be described now assuming that the individual components are in the position shown in FIG. 1. In this position, one row of type is aligned with the print hammers and the print drum 1 must be angularly driven in an intermittent, stepwise manner through 16 equal increments before that one row of type again reaches the position shown in FIG. 1 and such defines one printing cycle. As the print drum 1 is intermittently advanced, different rows of type align with the print hammers and when the correct row of type is in the print position, the print hammers are actuated to effect a printing operation.

As seen in FIG. 1, the drive lever 4 is urged into its initial limit position by the biasing spring 9 since the electromagnet 13 is not energized. In this initial position, the camming projection 15 coacts with the cam follower 18 to cam the detecting lever 16 in a clockwise direction about the pin 17 to turn the switch 20 OFF thereby opening the circuit connected to the counter When a print command signal 24 is fed to the flipflop circuit 25, a gate signal is fed to the AND gate 23 to open the gate whereupon the output pulse signal from the pulse signal generator 22 is applied to the electromagnet 13. The first pulse signal passing through the gate 23 energizes the electromagnet 13 to momentarily create therearound a magnetic field which coacts with armature 12 to effect clockwise pivotal movement of the drive lever 4 to angularly displace the ratchet wheel 3 a distance equal to one tooth. Thus the print drum 1 along with the cam 14 are displaced through an angle equal to one pitch.

Since the camming projection 15 subtends an angle equal to two pitch, the cam follower 18 remains upon the camming projection 15 and the switch 20 remains turned OFF even though the print drum has advanced one/sixteenth of a revolution and consequently, the counter 26 remains turned OFF. After the expiration of the first pulse, the magnetic field collapses and the attractive force between the armature 12 and the electromagnet 13 diminishes whereupon the biasing spring 9 returns the drive lever 4 to the initial position shown in FIG. 1. When the second pulse signal is delivered through the gate 23 and is applied to the electromagnet 13, the aforementioned cycle of operation is repeated and the print drum 1 is advanced another sixteenth of a revolution.

As the cam 14 is angularly displaced another pitch, the cam follower 18 slides down off of the camming projection 15 whereupon the biasing spring 19 pivots the detecting lever 16 in a counter clockwise direction maintaining the cam follower 18 in camming contact with the cam 14. As the detecting lever 16 is pivoted, the actuating-end of the lever is brought out of engagement with the movable contact pin of the switch 20 thereby turning the switch 20 ON. As soon as the switch 20 is turned ON, the counter 26 is turned ON and will subsequently count the remaining pulse signals delivered from the AND gate 23. Thus the third pulse signal through the sixteenth pulse signal applied to the electromagnet 13 will be counted by the counter and the application of these pulses will intermittently and angularly displace the print drum 1 back to its initial position. During the time period that any one of the rows of type are aligned with the print hammers, the print hammers may be selectively actuated to effect a printing operation.

After the counter 26 has counted fourteen pulse signals, it delivers a reset signal to the flip-flop circuit 25. The flip-flop circuit 25 is reset by the reset signal and stops supplying a gate signal to the AND gate 23 and consequently the output signals from the signal generator 22 are blocked by the AND gate 23 and do not reach the electromagnet 13. During angular displacement of the print drum 1 through the sixteenth pitch, the cam follower l8 rides up the leading edge of the camming projection 15 and upon the completion of 16 pitches or one revolution of the print drum, the components assume the positions shown in FIG. 1 and the actuating-end of the detecting lever 16 again engages the movable contact pin switch 20 thereby turning the switch OFF thereby turning OFF the counter 26.

In the embodiment shown, the camming projection 15 has a camming width corresponding to two pitches of the teeth on the ratchet wheel 3 and the detecting means A detects when the print drum 1 has completed one revolution. In an alternative embodiment, the camming projection 15 may have a different camming width and thus may detect when the printdrum 1 has been driven through any desired angular distance.

In accordance with the present invention, the problem of missing an intended printing operation is prevented. If for any reason the electric power to the device is interrupted, the stopped state of the printing mechanism is not the starting state for the next printing operation. Obviously, if the next printing operation commenced from the stopped state, misprinting will occur since the print drum 1 lags behind its initial starting position. This problem is obviated in the present invention by having the counter 26 actuated by the detecting means A. The counter 26 only begins to count the pulse signals when the print drum 1 is at a prescribed angular position and a certain number of pulses must be counted by the counter 26 before same applies the reset signal to the flip-flop 25 to place the flip-flop in condition to receive another print command signal.

In the embodiment shown, the counter 26 begins to count pulses only when the switch 20 is turned ON. In another embodiment, the counter 26 can be turned ON in response to turning OFF the switch 20. Also, the

camming width of the camming projection may be reduced to be less then one pitch whereby the counter 26 may be actuated when the detecting lever 16 engages with the camming projection rather than when the detecting lever slides off of the camming projection. In addition, the detecting means A may comprise a pressure-sensitive element, a magnetic-responsive device, optical means are the like in place of the combination of the cam 14, detecting lever 16, and the mechanical switch 20in order to detect the angular position of the print drum 1.

The invention has been described in conjunction with one particular embodiment and it is to be understood that obvious modifications and changes may be made without departing from the spirit and scope of the invention as defined in the appended claims.

What we claim is:

1. A printing mechanism comprising:

a rotatable print drum containing around the periphery thereof a plurality of circumferentially spacedapart sets of print type; drive means responsive to a succession pulse signals for rotationally driving said print drum in an intermittent manner through a succession of angular increments each corresponding to one pulse signal, said drive means comprising a ratchet wheel connected to said print drum for rotational movement therewith, a pawl coacting with said ratchet wheel to effect intermittent stepwise rotation of said ratchet wheel in response to recoprocatory movement of said pawl, and pviotal driving lever connected to said pawl, an armature affixed to said driving lever, and a electromagnet responsive to each pulse signal to create therearound a magnetic field having sufficient strength to attract said armature towards said electromagnet accompanied by pivotal movement of said driving lever and paw] to effect angular movement of said print drum through one angular increment; detecting means for detecting when said drum has been angularly driven to a predetermined angular start position during each rotational cycle and providing a corresponding output signal; and a control circuit operative to develop a succession of pulse signals and apply same to said electromagnet to effect intermittent rotation of said print drum and inclding means responsive to said output signal for terminating the application of the pulse signal to said electromagnet after said print drum has been driven through a prescribed nubmer of angular increments and returned to said start position, said control circuit comprising a gate circuit receptive of said succession of pulse signals and operative when in a gating state to gate the pulse signals to said electromagnet and operative when in a blocking state to block application of the pulse signals to said electromagnet, and counting means receptive of the pulse signals applied to said electromagnet and operative in response to said output signal for counting a predetermined number of pulse signals and then delivering a reset signal to said gate circuit to place same in said blocking state.

2. A printing mechanism according to claim 1 wherein said gate circuit comprises an AND gate for gating the pulse signals to said drive means, and flipflop circuit responsive to said output signal for controlling said AND gate. 5

3. A printing mechanism according to claim 1; wherein said plurality of sets of print type comprises n number of circumferentially spaced-apart and longtiudinally extending rows of print type.

4. A printing mechanism according to claim 1; wherein said detecting means comprises a cam connected to said print drum for rotational movement therewith and having a camming projection, and means coacting with said camming projection for providing said output signal when said print drum reaches said predetermined angular position.

5. A printing mechanism according to claim 4, wherein said last-mentioned means comprises a switch having a normal OFF position wherein same provides no output signal and an ON position wherein same provides said output signal, and a pivotal lever having a cam follower on one portion in camming contact with said cam and having another portion engageable with said switch to actuate same to said ON position when said cam follower rides'off said camming projection.

6. A printing meachism accordingto claim 4 wherein said electromagnetic drive means comprises a ratchet wheel connected to said print drum for rotational movement therewith, a pawl coacting with said ratchet wheel to effect inermittent stepwise rotation of said ratchet wheel in response to reciprocatory movement of said pawl, a pivotal driving lever connected to said pawl, an armature affixed to said driving lever, and an electromagnet responsive to each pulse signal to create therearound a magnetic field having sufficient strength to attract said armature towards said electromagnet accompanied by pivotal movement of said drivng lever and pawl to effect angular movement of said print drum through one angular increment.

7. A printing mechanism according to claim 6; wherein said camming projection subtends a central angle equal to a whole nubmer multiple of the number of ratchet teeth on said ratchet wheel. 

1. A printing mechanism comprising: a rotatable print drum containing around the periphery thereof a plurality of circumferentially spaced-apart sets of print type; drive means responsive to a succession pulse signals for rotationally driving said print drum in an intermittent manner through a succession of angular increments each corresponding to one pulse signal, said drive means comprising a ratchet wheel connected to said print drum for rotational movement therewith, a pawl coacting with said ratchet wheel to effect intermittent stepwise rotation of said ratchet wheel in response to recoprocatory movement of said pawl, and pviotal driving lever connected to said pawl, an armature affixed to said driving lever, and a electromagnet responsive to each pulse signal to create therearound a magnetic field having sufficient strength to attract said armature towards said electromagnet accompanied by pivotal movement of said driving lever and pawl to effect angular movement of said print drum through one angular increment; detecting means for detecting when said drum has been angularly driven to a predetermined angular start position during each rotational cycle and providing a corresponding output signal; and a control circuit operative to develop a succession of pulse signals and apply same to said electromagnet to effect intermittent rotation of said print drum and inclding means responsive to said output signal for terminating the application of the pulse signal to said electromagnet after said print drum has been driven through a prescribed nubmer of angular increments and returned to said start position, said control circuit comprising a gate circuit receptive of said succession of pulse signals and operative when in a gating state to gate the pulse signals to said electromagnet and operative when in a blocking state to block application of the pulse signals to said electromagnet, and counting means receptive of the pulse signals applied to said electromagnet and operative in response to said output signal for counting a predetermined number of pulse signals and then delivering a reset signal to said gate circuit to place same in said blocking state.
 2. A printing mechanism according to claim 1 wherein said gate circuit comprises an AND gate for gating the pulse signals to said drive means, and flip-flop circuit responsive to said output signal for controlling said AND gate.
 3. A printing mechanism according to claim 1; wherein said plurality of sets of print type comprises n number of circumferentially spaced-apart and longtiudinally extending rows of print type.
 4. A printing mechanism according to claim 1; wherein said detecting means comprises a cam connected to said print drum for rotational movement therewith and having a camming projection, and means coacting with said camming projection for providing said output signal when said print drum reaches said predetermined angular position.
 5. A printing mechanism according to claim 4, wherein said last-mentioned means comprises a switch having a normal OFF position wherein same provides no output signal and an ON position wherein same provides said output signal, and a pivotal lever having a cam follower on one portion in camming contact with said cam and having another portion engageable with said switch to actuate same to said ON position when said cam follower rides off said camming projection.
 6. A printing meachism according to claim 4 wherein said electromagnetic drive means comprises a ratchet wheel connected to said print drum for rotational movement therewith, a pawl coacting with said ratchet wheel to effect inermittent stepwise rotation of said ratchet wheel in response to reciprocatory movement of said pawl, a pivotal driving lever connected to said pawl, an armature affixed to said driving lever, and an electromagnet responsive to each pulse signal to create therearound a magnetic field having sufficient strength to attract said armature towards said electromagnet accompanied by pivotal movement of said drivng lever and pawl to effect angular movement of said print drum through one angular increment.
 7. A printing mechanism according to claim 6; wherein said camming projection subtends a central angle equal to a whole nubmer multiple of the number of ratchet teeth on said ratchet wheel. 